Nowadays, microlithographic projection exposure methods are predominantly used for producing semiconductor components and other finely structured devices. In this case, masks (reticles) are used which bear the pattern of a structure to be imaged, e.g., a line pattern of a layer of a semiconductor component. A mask is positioned into a projection exposure apparatus between an illumination system and a projection objective in the region of the object surface of the projection objective and is illuminated with an illumination radiation provided by the illumination system. The radiation altered by the mask and the pattern passes as projection radiation through the projection objective, which images the pattern of the mask onto the substrate to be exposed, which normally bears a radiation-sensitive layer (photoresist).
Current projection exposure apparatuses for high-resolution microlithography in the deep or very deep ultraviolet range (DUV or VUV) generally use a laser as primary light source. In particular, KrF excimer lasers having an operating wavelength of approximately 248 nm or ArF excimer lasers having an operating wavelength of approximately 193 nm are conventional. F2 lasers are employed for an operating wavelength of 157 nm, and Ar2 excimer lasers can be used at 126 nm. The primary laser radiation source emits a laser beam composed of laser light, which is received by the connected illumination system and reshaped in order to generate an illumination radiation directed onto the mask. The spectral properties of the illumination radiation and of the projection radiation, that is to say their properties dependent on the wavelength λ or the angular frequency ω, are in this case substantially determined by the spectral properties of the primary laser radiation.
In the design of a lithography process, line widths of the structures on the reticle are adapted in such a way that after imaging with the aid of the projection objective using an illumination assumed to be known, the desired structure sizes are exposed in the light-sensitive layer. In this case, it is important for identical structures of the mask to be imaged identically in the photoresist independently of the location on the substrate. Otherwise, in the case of semiconductor components, price-reducing losses of speed or in the worst case even functional losses can occur. One critical variable in semiconductor production, therefore, is the change in the thickness of the critical structures (CD) which is brought about by the process, and which is also designated as “variation of the critical dimensions” or “CD variation”. Accordingly, a uniform width of imaged identical structures over the field, so-called CD uniformity, constitutes an essential quality criterion of lithography processes.
A determining factor for the width of a structure in the photoresist is the radiation energy deposited there. It is assumed to a customary approximation that the photoresist is exposed above a specific deposited amount of radiation energy, and is not exposed below that amount. The limit value for the amount of radiation energy is also designated as “resist threshold”. What is important in this case is the radiation intensity integrated during a total exposure time at a location on the substrate. The magnitude of the radiation energy deposited at a specific location in the photoresist is dependent on a large number of influencing variables, inter alia on optical aberrations, in particular on chromatic aberrations, on the polarization state of the exposure radiation and also on the influence of stray light and double reflections. If a laser is used as primary light source, then the influence of so-called speckles, which can arise when using at least partially coherent radiation as a result of self-interference, can be added as a further potential cause of CD variations. So-called temporally varying speckles (dynamic speckles, temporal speckles) are primarily important here, these being caused by temporal intensity fluctuations resulting from the fact that use is made of a light source having a coherence time which is not very much shorter than the duration of a laser pulse. By contrast, the coherence time of typical incoherent light sources is so short that temporally varying speckles are not a problem in that case.
Numerous influencing parameters should be taken into consideration when selecting suitable laser radiation sources for a microlithography projection exposure apparatus. Particularly in the case of high-resolution projection objectives with a large image field, chromatic correction is very complex. If a projection objective is not fully chromatically corrected, then radiation having different wavelengths produces for each wavelength a different focal position in the image field of the projection objective. In order to avoid the resultant disadvantages it is generally endeavored to use very narrowband laser radiation sources. Conventional excimer lasers therefore contain bandwidth narrowing modules which narrow the natural emission spectrum of the laser of a few hundred μm by several powers of ten, for example to bandwidths of less than 1 pm. Large bandwidths are thus disadvantageous with regard to the chromatic aberrations. By contrast, large bandwidths are rather expedient with regard to the arising of dynamic speckles, since the undesirable interference phenomena designated as speckles are attributable to the temporal coherence of the light. Since the light is less coherent, the more different wavelengths are contained in the laser radiation, large bandwidths can be expedient with regard to avoiding speckles.
Therefore, the selection of a suitable bandwidth is generally a compromise between the requirements for low chromatic aberrations, on the one hand, and few speckles, on the other hand. The setting of the optimum bandwidth is a technological question that has to be decided when designing each projection exposure apparatus on the basis of the available data, for example for the chromatic correction of the projection objective.